Eight victims have contacted West Midlands Police in recent months after being instructed to pay thousands of pounds into offshore bank accounts.

Officers say the blackmailers have lured men into online video sex chats and then sent screen grabs to them along with demands for cash.

The recent spike in offences has prompted detectives to issue a warning for people to be vigilant.

One of the men faced a demand for £3,000 from offenders who threatened to email images to his girlfriend and his daughter.

The offenders usually target their victims by adding them as a friend on social media sites such as Facebook, or through conversations on Skype or FaceTime.

Detective Constable Stefan Ashton, of West Midlands Police CID, has been looking into the reports.

"Since January we've seen a rise in the number of these offences in the West Midlands, with victims in Wolverhampton, Birmingham, Dudley, Walsall and Coventry," he said.

"In most of the cases the videos have not actually been distributed but it's nevertheless been an absolutely horrendous ordeal for the people involved, who are understandably very worried about the consequences of such images and videos being posted online.

"The damage in the cases where the footage was shared with the victims' Facebook friends cannot be underestimated - they are clearly devastated.

"The blackmailers are calculated and malicious and have complete disregard for the people they're exploiting."

Det Con Ashton added: "Cases of this type are notoriously complex and because IP addresses are usually traced back to foreign countries, they often, despite our best efforts, go unsolved.

"We are working closely with partners such as the National Crime Agency to crack down on offenders but the only real way to prevent this from happening is by not taking part in such activity at all.

"The moment you reveal yourself in an online video, you become vulnerable to exploitation and sadly it's the blackmailers who instantly become in control of the situation."